The present invention relates, in general, to pin grid array (PGA) packages for integrated circuits, and more particularly, to a pin grid array package which is easily provided with a low reflection line.
Pin grid array packages are widely used in packaging very large scale integrated circuits. One such pin grid array package is disclosed in U.S. Pat. No. 4,513,355 which issued on Apr. 23, 1985 to Schroeder et al. This U.S. Patent (U.S. Pat. No. 4,513,355) is hereby incorporated herein by reference. A PGA package has a plurality of terminals, or pins, which are arranged in a grid on a substrate. Usually the substrate is a multi-layer sandwich of dielectric material and patterned metal layers. The substrate usually has a centrally located opening in which an integrated circuit is mounted. Conductive lines electrically couple the PGA package pins with the integrated circuit. Because multiple layers of metal were used in the PGA package, the conductive lines formed a stripline conductor, wherein the conductive lines were sandwiched between two ground planes and isolated from the ground planes by a layer of dielectric material. The stripline conductors are also called signal lines, or transmission lines. The stripline conductor design reduces signal distortion as a signal travels from the package to the integrated circuit, but due to the difficulties in adding terminating resistors the stripline conductors were not terminated, which resulted in large reflection voltages when signals had fast edge rates.
To achieve higher operating frequencies, clock signals and data signals must have faster edge rates. To minimize distortion, signals having fast edge rates are conducted external to the semiconductor package on signal lines having a constant characteristic impedance Z.sub.0, which is usually 50 ohms, and the signal lines are terminated with an impedance which matches the signal line. This is because when a signal having a fast edge rate is transmitted on a signal line with variable impedance, or which is not terminated with a matched impedance, at each impedance mismatch a portion of the signal is reflected toward the signal source, and a portion is transmitted forward on the signal line. The magnitude of the reflective voltage is a function of the edge rate of the signal and the capacitance of the transmission line termination. The terminating resistance is coupled between the signal line and ground reference or a negative or positive power supply. A signal is coupled to a receiving gate, also called a receiving line, of an integrated circuit by a branch signal line coupled to the terminated transmission line. Unterminated signal lines which branch from a terminated transmission line will also cause reflected voltages, the magnitude of which is a function of signal edge rate and capacitance of the unterminated branch.
Reflected signals from the PGA package signal line caused several problems. First, the wave shape of the signal that reached the receiving gate was distorted because part of the signal had been reflected, resulting in slower rise times and lower operating frequency. Second, the reflected portion of the signal appeared as noise on the external signal line, reducing the noise margin of data on that line. Third, the reflected wave caused standing waves in the signal line, which caused the signal amplitude to increase and decrease depending on capacitance of the package and signal frequency, further reducing the noise margin. All of these problems were aggravated when signal edge rates increased. Eventually the receiving gate did not operate reliably because of the reduced noise margin.
A solution to this problem was proposed in co-pending U S. patent application Ser. No. 374,791, filed July 3, 1989 by Prioste et al. This patent application, incorporated herein by reference, discloses a pin grid array package having dedicated low reflection signal lines which must be used for signals having fast edge rates. The low reflection lines provided by Prioste et al provide at least twice the system frequency bandwidth as compared with conventional PGA package traces or conductive lines. However, this approach requires that each special PGA package low reflection line requires two package pins that have to be designed and fabricated into the package. Because of the difference in the interconnect scheme for these special conductive lines as compared with conventional lines the packages become very customized to the specific applications and to the specific chips in the package. The number and location of these special low reflection lines are fixed once the packages have been designed and manufactured. Hence both chip design and system layout have to be tailored to the packages which may not always be cost effective.
Accordingly, an object of the present invention is to provide an improved PGA package which provides low reflection lines in a flexible manner.
Another object of the present invention is to provide a conventional PGA package having conductive lines which can readily be converted to low reflection lines.
A further object of the present invention is to provide a conventional PGA package capable of handling high speed microwave signals without substantially deteriorating the high speed microwave signals.